Long Term Evolution (LTE)
Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radioaccess technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies, 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support for the next decade. The ability to provide high bit rates is a key measure for LTE.
The work item (WI) specification on Long-Term Evolution (LTE) called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is finalized as Release 8 (LTE Rel. 8). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP) and its affinity to different transmission bandwidth arrangements. Single-carrier frequency division multiple access (SC-FDMA) based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmit power of the user equipment (UE). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques and a highly efficient control signaling structure is achieved in LTE Rel. 8/9.
LTE Architecture
The overall architecture is shown in FIG. 1 and a more detailed representation of the E-UTRAN architecture is given in FIG. 2. The E-UTRAN consists of an eNodeB, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink Quality of Service (QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface.
The eNodeBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (SGW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNodeBs. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle state user equipments, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the user equipment. It manages and stores user equipment contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode user equipment tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a user equipment at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to user equipments. It checks the authorization of the user equipment to camp on the service provider's Public Land Mobile Network (PLMN) and enforces user equipment roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming user equipments.
Component Carrier Structure in LTE (Release 8)
The downlink component carrier of a 3GPP LTE (Release 8) is subdivided in the time-frequency domain in so-called subframes. In 3GPP LTE (Release 8) each subframe is divided into two downlink slots as shown in FIG. 3, wherein the first downlink slot comprises the control channel region (PDCCH region) within the first OFDM symbols. Each subframe consists of a give number of OFDM symbols in the time domain (12 or 14 OFDM symbols in 3GPP LTE (Release 8)), wherein each OFDM symbol spans over the entire bandwidth of the component carrier. The OFDM symbols thus each consists of a number of modulation symbols transmitted on respective NRBDL×NscRB subcarriers as also shown in FIG. 4.
Assuming a multi-carrier communication system, e.g. employing OFDM, as for example used in 3GPP Long Term Evolution (LTE), the smallest unit of resources that can be assigned by the scheduler is one “resource block”. A physical resource block is defined as NsymbDL consecutive OFDM symbols in the time domain and NscRB consecutive subcarriers in the frequency domain as exemplified in FIG. 4. In 3GPP LTE (Release 8), a physical resource block thus consists of NsymbDL×NscRB resource elements, corresponding to one slot in the time domain and 180 kHz in the frequency domain (for further details on the downlink resource grid, see for example 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, version 8.9.0 or 9.0.0, section 6.2, available at http://www.3gpp.org and incorporated herein by reference).
The term “component carrier” refers to a combination of several resource blocks. In future releases of LTE, the term “component carrier” is no longer used; instead, the terminology is changed to “cell”, which refers to a combination of downlink and optionally uplink resources. The linking between the carrier frequency of the downlink resources and the carrier frequency of the uplink resources is indicated in the system information transmitted on the downlink resources.
Further Advancements for LTE (LTE-A)
The frequency spectrum for IMT-Advanced was decided at the World Radiocommunication Conference 2007 (WRC-07). Although the overall frequency spectrum for IMT-Advanced was decided, the actual available frequency bandwidth is different according to each region or country. Following the decision on the available frequency spectrum outline, however, standardization of a radio interface started in the 3rd Generation Partnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the Study Item description on “Further Advancements for E-UTRA (LTE-Advanced)” was approved. The study item covers technology components to be considered for the evolution of E-UTRA, e.g. to fulfill the requirements on IMT-Advanced. Two major technology components are described in the following.
Carrier Aggregation in LTE-a for Support of Wider Bandwidth
The bandwidth that the LTE-Advanced system is able to support is 100 MHz, while an LTE system can only support 20 MHz. Nowadays, the lack of radio spectrum has become a bottleneck of the development of wireless networks, and as a result it is difficult to find a spectrum band which is wide enough for the LTE-Advanced system. Consequently, it is urgent to find a way to gain a wider radio spectrum band, wherein a possible answer is the carrier aggregation functionality.
In carrier aggregation, two or more component carriers (component carriers) are aggregated in order to support wider transmission bandwidths up to 100 MHz. Several cells in the LTE system are aggregated into one wider channel in the LTE-Advanced system which is wide enough for 100 MHz even though these cells in LTE are in different frequency bands.
All component carriers can be configured to be LTE Rel. 8/9 compatible, at least when the aggregated numbers of component carriers in the uplink and the downlink are the same. Not all component carriers aggregated by a user equipment may necessarily be Rel. 8/9 compatible. Existing mechanism (e.g. barring) may be used to avoid Rel-8/9 user equipments to camp on a component carrier.
A user equipment may simultaneously receive or transmit one or multiple component carriers (corresponding to multiple serving cells) depending on its capabilities. A LTE-A Rel. 10 user equipment with reception and/or transmission capabilities for carrier aggregation can simultaneously receive and/or transmit on multiple serving cells, whereas an LTE Rel. 8/9 user equipment can receive and transmit on a single serving cell only, provided that the structure of the component carrier follows the Rel. 8/9 specifications.
Carrier aggregation is supported for both contiguous and non-contiguous component carriers with each component carrier limited to a maximum of 110 Resource Blocks in the frequency domain using the 3GPP LTE (Release 8/9) numerology.
It is possible to configure a 3GPP LTE-A (Release 10) compatible user equipment to aggregate a different number of component carriers originating from the same eNodeB (base station) and of possibly different bandwidths in the uplink and the downlink. The number of downlink component carriers that can be configured depends on the downlink aggregation capability of the UE. Conversely, the number of uplink component carriers that can be configured depends on the uplink aggregation capability of the UE. It may not be possible to configure a mobile terminal with more uplink component carriers than downlink component carriers.
In a typical TDD deployment, the number of component carriers and the bandwidth of each component carrier in uplink and downlink is the same. Component carriers originating from the same eNodeB need not to provide the same coverage.
The spacing between centre frequencies of contiguously aggregated component carriers shall be a multiple of 300 kHz. This is in order to be compatible with the 100 kHz frequency raster of 3GPP LTE (Release 8/9) and at the same time preserve orthogonality of the subcarriers with 15 kHz spacing. Depending on the aggregation scenario, the n×300 kHz spacing can be facilitated by insertion of a low number of unused subcarriers between contiguous component carriers.
The nature of the aggregation of multiple carriers is only exposed up to the MAC layer. For both uplink and downlink there is one HARQ entity required in MAC for each aggregated component carrier. There is (in the absence of SU-MIMO for uplink) at most one transport block per component carrier. A transport block and its potential HARQ retransmissions need to be mapped on the same component carrier.
The Layer 2 structure with activated carrier aggregation is shown in FIG. 5 and FIG. 6 for the downlink and uplink respectively.
When carrier aggregation is configured, the mobile terminal only has one RRC connection with the network. At RRC connection establishment/re-establishment, one cell provides the security input (one ECGI, one PCI and one ARFCN) and the non-access stratum mobility information (e.g. TAI) similarly as in LTE Rel. 8/9. After RRC connection establishment/re-establishment, the component carrier corresponding to that cell is referred to as the downlink Primary Cell (PCell). There is always one and only one downlink PCell (DL PCell) and one uplink PCell (UL PCell) configured per user equipment in connected state. Within the configured set of component carriers, other cells are referred to as Secondary Cells (SCells). The characteristics of the downlink and uplink PCell are:                The uplink PCell is used for transmission of Layer 1 uplink control information        The downlink PCell cannot be de-activated, unlike SCells        Re-establishment is triggered when the downlink PCell experiences Rayleigh fading (RLF), not when downlink SCells experience RLF        The downlink PCell cell can change with handover        Non-access stratum information is taken from the downlink PCell        PCell can only be changed with handover procedure (i.e. with security key change and RACH procedure)        PCell is used for transmission of PUCCH        
The configuration and reconfiguration of component carriers can be performed by RRC. Activation and deactivation is done via MAC control elements. At intra-LTE handover, RRC can also add, remove, or reconfigure SCells for usage in the target cell. When adding a new SCell, dedicated RRC signaling is used for sending the system information of the SCell, the information being necessary for transmission/reception (similarly as in Rel-8/9 for handover).
When a user equipment is configured with carrier aggregation there is one pair of uplink and downlink component carriers that is always active. The downlink component carrier of that pair might be also referred to as ‘DL anchor carrier’. Same applies also for the uplink.
When carrier aggregation is configured, a user equipment may be scheduled over multiple component carriers simultaneously but at most one random access procedure shall be ongoing at any time. Cross-carrier scheduling allows the PDCCH of a component carrier to schedule resources on another component carrier. For this purpose a component carrier identification field is introduced in the respective DCI formats, called CIF.
A linking between uplink and downlink component carriers allows identifying the uplink component carrier for which the grant applies when there is no-cross-carrier scheduling. The linkage of downlink component carriers to uplink component carrier does not necessarily need to be one to one. In other words, more than one downlink component carrier can link to the same uplink component carrier. At the same time, a downlink component carrier can only link to one uplink component carrier.
LTE RRC States
LTE is based on only two main states: “RRC_IDLE” and “RRC_CONNECTED”.
In RRC_IDLE the radio is not active, but an ID is assigned and tracked by the network. More specifically, a mobile terminal in RRC_IDLE performs cell selection and reselection—in other words, it decides on which cell to camp. The cell (re)selection process takes into account the priority of each applicable frequency of each applicable Radio Access Technology (RAT), the radio link quality and the cell status (i.e. whether a cell is barred or reserved). An RRC_IDLE mobile terminal monitors a paging channel to detect incoming calls, and also acquires system information. The system information mainly consists of parameters by which the network (E-UTRAN) can control the cell (re)selection process. RRC specifies the control signalling applicable for a mobile terminal in RRC_IDLE, namely paging and system information. The mobile terminal behaviour in RRC_IDLE is specified in TS 25.912, e.g. Chapter 8.4.2 incorporate herein by reference.
In RRC_CONNECTED the mobile terminal has an active radio operation with contexts in the eNodeB. The E-UTRAN allocates radio resources to the mobile terminal to facilitate the transfer of (unicast) data via shared data channels. To support this operation, the mobile terminal monitors an associated control channel which is used to indicate the dynamic allocation of the shared transmission resources in time and frequency. The mobile terminal provides the network with reports of its buffer status and of the downlink channel quality, as well as neighbouring cell measurement information to enable E-UTRAN to select the most appropriate cell for the mobile terminal. These measurement reports include cells using other frequencies or RATs. The UE also receives system information, consisting mainly of information required to use the transmission channels. To extend its battery lifetime, a UE in RRC_CONNECTED may be configured with a Discontinuous Reception (DRX) cycle. RRC is the protocol by which the E-UTRAN controls the UE behaviour in RRC_CONNECTED.
FIG. 7 shows a state diagram with an overview of the relevant functions performed by the mobile terminal in IDLE and CONNECTED state.
Layer 1/Layer 2 (L1/L2) Control Signaling
In order to inform the scheduled users about their allocation status, transport format and other data-related information (e.g. HARQ information, transmit power control (TPC) commands), L1/L2 control signaling is transmitted on the downlink along with the data. L1/L2 control signaling is multiplexed with the downlink data in a subframe, assuming that the user allocation can change from subframe to subframe. It should be noted that user allocation might also be performed on a TTI (Transmission Time Interval) basis, where the TTI length is a multiple of the sub-frames. The TTI length may be fixed in a service area for all users, may be different for different users, or may even by dynamic for each user. Generally, the L1/2 control signaling needs only be transmitted once per TTI.
The L1/L2 control signaling is transmitted on the Physical Downlink Control Channel (PDCCH). A PDCCH carries a message as a Downlink Control Information (DCI), which includes resource assignments and other control information for a mobile terminal or groups of UEs. In general, several PDCCHs can be transmitted in one subframe.
It should be noted that in 3GPP LTE, assignments for uplink data transmissions, also referred to as uplink scheduling grants or uplink resource assignments, are also transmitted on the PDCCH.
With respect to scheduling grants, the information sent on the L1/L2 control signaling may be separated into the following two categories, Shared Control Information (SCI) carrying Cat 1 information and Downlink Control Information (DCI) carrying Cat 2/3 information.
Shared Control Information (SCI) Carrying Cat 1 Information
The shared control information part of the L1/L2 control signaling contains information related to the resource allocation (indication). The shared control information typically contains the following information:                A user identity indicating the user(s) that is/are allocated the resources.        RB allocation information for indicating the resources (Resource Blocks (RBs)) on which a user(s) is/are allocated. The number of allocated resource blocks can be dynamic.        The duration of assignment (optional), if an assignment over multiple sub-frames (or TTIs) is possible.        
Depending on the setup of other channels and the setup of the Downlink Control Information (DCI)—see below—the shared control information may additionally contain information such as ACK/NACK for uplink transmission, uplink scheduling information, information on the DCI (resource, MCS, etc.).
Downlink Control Information (DCI) Carrying Cat 2/3 Information
The downlink control information part of the L1/L2 control signaling contains information related to the transmission format (Cat 2 information) of the data transmitted to a scheduled user indicated by the Cat 1 information. Moreover, in case of using (Hybrid) ARQ as a retransmission protocol, the Cat 2 information carries HARQ (Cat 3) information. The downlink control information needs only to be decoded by the user scheduled according to Cat 1. The downlink control information typically contains information on:                Cat 2 information: Modulation scheme, transport-block (payload) size or coding rate, MIMO (Multiple Input Multiple Output)-related information, etc. Either the transport-block (or payload size) or the code rate can be signaled. In any case these parameters can be calculated from each other by using the modulation scheme information and the resource information (number of allocated resource blocks)        Cat 3 information: HARQ related information, e.g. hybrid ARQ process number, redundancy version, retransmission sequence number        
Downlink control information occurs in several formats that differ in overall size and also in the information contained in its fields. The different DCI formats that are currently defined for LTE are as follows and described in detail in 3GPP TS 36.212, “Multiplexing and channel coding”, section 5.3.3.1 (available at http://www.3gpp.org and incorporated herein by reference).    Format 0: DCI Format 0 is used for the transmission of resource grants for the PUSCH.    Format 1: DCI Format 1 is used for the transmission of resource assignments for single codeword PDSCH transmissions (transmission modes 1, 2 and 7).    Format 1A: DCI Format 1A is used for compact signaling of resource assignments for single codeword PDSCH transmissions, and for allocating a dedicated preamble signature to a mobile terminal for contention-free random access.    Format 1B: DCI Format 1B is used for compact signaling of resource assignments for PDSCH transmissions using closed loop precoding with rank-1 transmission (transmission mode 6). The information transmitted is the same as in Format 1A, but with the addition of an indicator of the precoding vector applied for the PDSCH transmission.    Format 1C: DCI Format 1C is used for very compact transmission of PDSCH assignments. When format 1C is used, the PDSCH transmission is constrained to using QPSK modulation. This is used, for example, for signaling paging messages and broadcast system information messages.    Format 1D: DCI Format 1D is used for compact signaling of resource assignments for PDSCH transmission using multi-user MIMO. The information transmitted is the same as in Format 1B, but instead of one of the bits of the precoding vector indicators, there is a single bit to indicate whether a power offset is applied to the data symbols. This feature is needed to show whether or not the transmission power is shared between two UEs. Future versions of LTE may extend this to the case of power sharing between larger numbers of UEs.    Format 2: DCI Format 2 is used for the transmission of resource assignments for PDSCH for closed-loop MIMO operation.    Format 2A: DCI Format 2A is used for the transmission of resource assignments for PDSCH for open-loop MIMO operation. The information transmitted is the same as for Format 2, except that if the eNodeB has two transmit antenna ports, there is no precoding information, and for four antenna ports two bits are used to indicate the transmission rank.    Format 3 and 3A: DCI formats 3 and 3A are used for the transmission of power control commands for PUCCh and PUSCH with 2-bit or 1-bit power adjustments respectively. These DCI formats contain individual power control commands for a group of UEs.
The following table gives an overview of the available DCI formats.
Number of bits includingCRC (for a systemDCIbandwidth of 50 RBs andformatPurposefour antennas at eNodeB0PUSCH grants421PDSCH assignments with a47single codeword1APDSCH assignments using42a compact format1BPDSCH assignments for rank-146transmission1CPDSCH assignments using a very26compact format1DPDSCH assignments for multi-46user MIMO2PDSCH assignments for closed-loop62MIMO operation2APDSCH assignments for open-loop58MIMO operation3Transmit Power Control (TPC)42commands for multiple users forPUCCH and PUSCH with 2-bitpower adjustments3ATransmit Power Control (TPC)42commands for multiple users forPUCCH and PUSCH with 1-bitpower adjustments
For further information regarding the DCI formats and the particular information that is transmitted in the DCI, please refer to the technical standard or to LTE—The UMTS Long Term Evolution—From Theory to Practice, Edited by Stefanie Sesia, Issam Toufik, Matthew Baker, Chapter 9.3, incorporated herein by reference.
Downlink & Uplink Data Transmission
Regarding downlink data transmission, L1/L2 control signaling is transmitted on a separate physical channel (PDCCH), along with the downlink packet data transmission. This L1/L2 control signaling typically contains information on:                The physical resource(s) on which the data is transmitted (e.g. subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA). This information allows the mobile terminal (receiver) to identify the resources on which the data is transmitted.        When user equipment is configured to have a Carrier Indication Field (CIF) in the L1/L2 control signaling, this information identifies the component carrier for which the specific control signaling information is intended. This enables assignments to be sent on one component carrier which are intended for another component carrier (“cross-carrier scheduling”). This other, cross-scheduled component carrier could be for example a PDCCH-less component carrier, i.e. the cross-scheduled component carrier does not carry any L1/L2 control signaling.        The Transport Format, which is used for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation (e.g. the number of resource blocks assigned to the user equipment)) allows the user equipment (receiver) to identify the information bit size, the modulation scheme and the code rate in order to start the demodulation, the de-rate-matching and the decoding process. The modulation scheme may be signaled explicitly.        Hybrid ARQ (HARQ) information:                    HARQ process number: Allows the user equipment to identify the hybrid ARQ process on which the data is mapped.            Sequence number or new data indicator (NDI): Allows the user equipment to identify if the transmission is a new packet or a retransmitted packet. If soft combining is implemented in the HARQ protocol, the sequence number or new data indicator together with the HARQ process number enables soft-combining of the transmissions for a PDU prior to decoding.            Redundancy and/or constellation version: Tells the user equipment, which hybrid ARQ redundancy version is used (required for de-rate-matching) and/or which modulation constellation version is used (required for demodulation).                        UE Identity (UE ID): Tells for which user equipment the L1/L2 control signaling is intended for. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other user equipments to read this information.        
To enable an uplink packet data transmission, L1/L2 control signaling is transmitted on the downlink (PDCCH) to tell the user equipment about the transmission details. This L1/L2 control signaling typically contains information on:                The physical resource(s) on which the user equipment should transmit the data (e.g. subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA).        When user equipment is configured to have a Carrier Indication Field (CIF) in the L1/L2 control signaling, this information identifies the component carrier for which the specific control signaling information is intended. This enables assignments to be sent on one component carrier which are intended for another component carrier. This other, cross-scheduled component carrier may be for example a PDCCH-less component carrier, i.e. the cross-scheduled component carrier does not carry any L1/L2 control signaling.        L1/L2 control signaling for uplink grants is sent on the DL component carrier that is linked with the uplink component carrier or on one of the several DL component carriers, if several DL component carriers link to the same UL component carrier.        The Transport Format, the user equipment should use for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation (e.g. the number of resource blocks assigned to the user equipment)) allows the user equipment (transmitter) to pick the information bit size, the modulation scheme and the code rate in order to start the modulation, the rate-matching and the encoding process. In some cases the modulation scheme maybe signaled explicitly.        Hybrid ARQ information:                    HARQ Process number: Tells the user equipment from which hybrid ARQ process it should pick the data.            Sequence number or new data indicator: Tells the user equipment to transmit a new packet or to retransmit a packet. If soft combining is implemented in the HARQ protocol, the sequence number or new data indicator together with the HARQ process number enables soft-combining of the transmissions for a protocol data unit (PDU) prior to decoding.            Redundancy and/or constellation version: Tells the user equipment, which hybrid ARQ redundancy version to use (required for rate-matching) and/or which modulation constellation version to use (required for modulation).                        UE Identity (UE ID): Tells which user equipment should transmit data. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other user equipments to read this information.        
There are several different possibilities how to exactly transmit the information pieces mentioned above in uplink and downlink data transmission. Moreover, in uplink and downlink, the L1/L2 control information may also contain additional information or may omit some of the information. For example:                HARQ process number may not be needed, i.e. is not signaled, in case of a synchronous HARQ protocol.        A redundancy and/or constellation version may not be needed, and thus not signaled, if Chase Combining is used (always the same redundancy and/or constellation version) or if the sequence of redundancy and/or constellation versions is pre-defined.        Power control information may be additionally included in the control signaling.        MIMO related control information, such as e.g. pre-coding, may be additionally included in the control signaling.        In case of multi-codeword MIMO transmission transport format and/or HARQ information for multiple code words may be included.        
For uplink resource assignments (on the Physical Uplink Shared Channel (PUSCH)) signaled on PDCCH in LTE, the L1/L2 control information does not contain a HARQ process number, since a synchronous HARQ protocol is employed for LTE uplink. The HARQ process to be used for an uplink transmission is given by the timing. Furthermore, it should be noted that the redundancy version (RV) information is jointly encoded with the transport format information, i.e. the RV info is embedded in the transport format (TF) field. The Transport Format (TF) respectively modulation and coding scheme (MCS) field has for example a size of 5 bits, which corresponds to 32 entries. 3 TF/MCS table entries are reserved for indicating redundancy versions (RVs) 1, 2 or 3. The remaining MCS table entries are used to signal the MCS level (TBS) implicitly indicating RV0. The size of the CRC field of the PDCCH is 16 bits.
For downlink assignments (PDSCH) signaled on PDCCH in LTE the Redundancy Version (RV) is signaled separately in a two-bit field. Furthermore the modulation order information is jointly encoded with the transport format information. Similar to the uplink case there is 5 bit MCS field signaled on PDCCH. 3 of the entries are reserved to signal an explicit modulation order, providing no Transport format (Transport block) info. For the remaining 29 entries modulation order and Transport block size info are signaled.
Physical Downlink Control Channel (PDCCH)
As already explained, a PDCCH carriers messages as DCIs. Each PDCCH is transmitted on an aggregation of one or more so called Control Channel Elements (CCEs), where each CCE corresponds to nine Resource Element Groups (REGs i.e. sets of four physical resource elements). REGs constituting CCEs are not consecutive and CCEs are distributed in frequency over entire bandwidth. Note that CCEs are spread in frequency domain to achieve frequency diversity. Four PDCCH formats are supported as listed in the following table, which also shows the corresponding possible CCE aggregation levels.
PDDCHformatNumber of CCEsNumber of REGsNumber of PDCCH bits01972121814424362883872576
CCEs are numbered and used consecutively, and to simplify the decoding process, a PDCCH with a format consisting of n CCEs may only start with a CCE with a number equal to a multiple of n.
The number of available CCEs in a cell varies; it may be semi static (System bandwith, PHICH configuration) or dynamic (PCFICH).
The number of CCEs used for transmission of a particular PDCCH is determined by the eNodeB according to channel conditions. For example, if the PDCCH is intended for a mobile terminal with a good downlink channel, (e.g. close to the eNodeB) then one CCE is likely to be sufficient. However, for a mobile terminal with a poor channel (e.g. near the cell border) then eight CCEs may be required in order to achieve sufficient robustness. In addition, the power level of a PDCCH may be adjusted to match the channel conditions.
In detecting a PDCCH, the mobile terminal shall monitor a set of PDCCH candidates for control information in every non-DRX subframe, where monitoring refers to the process of attempting to decode each of PDCCHs in the set according to all DCI formats, as will be explained in more detail later.
In order that the mobile terminal can identify whether it has received a PDDCH transmission correctly, error detection is provided by means of a 16-bit CRC appended to each PDCCH. Furthermore, it is necessary that the UE can identify which PDCCHs are intended for it. This could in theory be achieved by adding an identifier to the PDCCH payload; however, it turns out to be more efficient to scramble the CRC with the UE identity (e.g. C-RNTI, Cell Radio Network Temporary Identifier), which saves the additional payload but at the cost of a small increase in the probability of falsely detecting a PDCCH intended for another UE.
The Physical Control Format Indicator Channel (PCFICH) carries a Control Format Indicator (CFI) which indicates the number of OFDM symbols used for transmission of control channel information in each subframe. The eNodeB is capable of transmitting multiple PDCCHs in a subframe. The transmissions are organized such that a UE can locate the PDCCHs intended for it, while at the same time making efficient use of the resources allocated for PDCCH transmissions.
A simple approach, at least for the eNodeB, would be to allow the eNodeB to place any PDCCH anywhere in the PDCCH resources (or CCEs) indicated by the PCFICH. In this case, the UE would need to check all possible PDCCH locations, PDCCH formats and DCI formats, and act on those messages with correct CRCs (taking into account the CRC is scrambled with a UE identity). Carrying out such a blind decoding of all the possible combinations would require the UE to make many PDDCH decoding attempts in every subframe. For small system bandwidths the computational load might be reasonable, but for large system bandwidths with a large number of possible PDCCH locations, it would become a significant burden, leading to excessive power consumption in the UE receiver.
The alternative approach adopted for LTE is to define for each UE a limited set of CCE locations where a PDCCH may be placed. Such a constraint may lead to some limitations as to which UEs can be sent PDCCHs within the same subframe, which would thus restrict the UEs to which the eNodeB could grant resources. Therefore, it is important for good system performance that the set of possible PDCCHs locations available for each UE is not too small. The set of CCE locations in which the UE may find its PDCCHs can be considered as a search space. In LTE the search space is of different size for each PDCCH format. Moreover, separate dedicated and common search spaces are defined, where a dedicated search space is configured for each UE individually, while all UEs are informed of the extent of the common search space. Note that the dedicated and common search spaces may overlap for a given UE.
With small search spaces it is quite possible in a given subframe that the eNodeB cannot find CCE resources to send PDCCHs to all the UEs that it would like to, because, having assigned some CCE locations, the remaining CCE locations are not in the search space of a particular UE.
In order to keep under control the computational load arising from the total number of blind decoding attempts, the UE is not required to search for all the defined DCI formats simultaneously.
In the common search space the UE will search for DCI Formats 1A and 1C. In addition, the UE may be configured to search for Format 3 or 3A, which have the same size as DCI formats 0 and 1A, and may be distinguished by having the CRC scrambled by different (common) identity (e.g. PC-PUCCH-RNTI), rather than a UE-specific one (e.g. C-RNTI). In particular, PC-PUCCH-RNTI (Transmit Power Control-Physical Uplink Control Channel-RNTI) and TPC-PUSCH-RNTI (Transmit Power Control-Physical Uplink Shared Channel-RNTI) are used in said respect.
DCI formats 0, 1A, 1C, 3 and 3A have two different payload sizes. The common search space is monitored by all UEs and may correspond to CCEs 0-15, rendering 4 decoding candidates with PDCCH format 2: 0-3, 4-7, 8-11, 12-15, or 2 decoding candidates with PDCCH format 3: 0-7, 8-15. In this case, there would be six blind decode attempts per payload size, and two different PDCCH payload sizes, thus having a total number of blind decodes per UE of 12.
The power-control message of DCI Format 3, 3A is directed to a group of terminals using an RNTI specific for that group. Each terminal can be allocated two power-control RNTIs, one for PUCCH power control and the other for PUSCH power control.
Typically, in the dedicated search space, the UE will always search for DCI formats 0 and 1A, which are both the same size and are distinguished by a flag in the message. In addition, a UE may be required to receive further DCI formats (e.g. 1, 1B or 2) depending on the PDSCH transmission mode configured by the eNodeB.
The starting location of the UE specific search space is usually determined by a hashing function based e.g. on the slot number within the radio frame, the RNTI value and other parameters. The UE specific search space allows aggregation levels of 1, 2, 4 and 8 CCEs.
Further information is provided in LTE—The UMTS Long Term Evolution—From Theory to Practice, Edited by Stefanie Sesia, Issam Toufik, Matthew Baker, Chapter 9.3, incorporated herein by reference.
DRX (Discontinuous Reception)
DRX functionality can be configured for RRC_IDLE, in which case the UE uses either the specific or default DRX value (defaultPagingCycle); the default is broadcasted in the System Information, and can have values of 32, 64, 128 and 256 radio frames. If both specific and default values are available, the shorter value of the two is chosen by the UE. The UE needs to wake up for one paging occasion per DRX cycle, the paging occasion being one subframe.
DRX functionality can be also configured for an “RRC_CONNECTED” UE, so that it does not always need to monitor the downlink channels. In order to provide reasonable battery consumption of user equipment, 3GPP LTE (Release 8/9) as well as 3GPP LTE-A (Release 10) provides a concept of discontinuous reception (DRX). Technical Standard TS 36.321 Chapter 5.7 explains the DRX and is incorporated by reference herein.
The following parameters are available to define the DRX UE behavior; i.e. the On-Duration periods at which the mobile node is active, and the periods where the mobile node is in a DRX mode.                On duration: duration in downlink sub-frames that the user equipment, after waking up from DRX, receives and monitors the PDCCH. If the user equipment successfully decodes a PDCCH, the user equipment stays awake and starts the inactivity timer; [1-200 subframes; 16 steps: 1-6, 10-60, 80, 100, 200]        DRX inactivity timer: duration in downlink sub-frames that the user equipment waits to successfully decode a PDCCH, from the last successful decoding of a PDCCH; when the UE fails to decode a PDCCH during this period, it re-enters DRX. The user equipment shall restart the inactivity timer following a single successful decoding of a PDCCH for a first transmission only (i.e. not for retransmissions). [1-2560 subframes; 22 steps, 10 spares: 1-6, 8, 10-60, 80, 100-300, 500, 750, 1280, 1920, 2560]        DRX Retransmission timer: specifies the number of consecutive PDCCH subframes where a downlink retransmission is expected by the UE after the first available retransmission time. [1-33 subframes, 8 steps: 1, 2, 4, 6, 8, 16, 24, 33]        DRX short cycle: specifies the periodic repetition of the on duration followed by a possible period of inactivity for the short DRX cycle. This parameter is optional. [2-640 subframes; 16 steps: 2, 5, 8, 10, 16, 20, 32, 40, 64, 80, 128, 160, 256, 320, 512, 640]        DRX short cycle timer: specifies the number of consecutive subframes the UE follows the short DRX cycle after the DRX Inactivity Timer has expired. This parameter is optional.[1-16 subframes]        Long DRX Cycle Start offset: specifies the periodic repetition of the on duration followed by a possible period of inactivity for the DRX long cycle as well as an offset in subframes when on-duration starts (determined by formula defined in TS 36.321 section 5.7); [cycle length 10-2560 subframes; 16 steps: 10, 20, 30, 32, 40, 64, 80, 128, 160, 256, 320, 512, 640, 1024, 1280, 2048, 2560; offset is an integer between [0-subframe length of chosen cycle]]        
The total duration that the UE is awake is called “Active time”. The Active Time includes the on-duration of the DRX cycle, the time UE is performing continuous reception while the inactivity timer has not expired and the time UE is performing continuous reception while waiting for a downlink retransmission after one HRQ RTT. Based on the above, the minimum active time is of length equal to on-duration, and the maximum is undefined (infinite).
The operation of DRX gives the mobile terminal the opportunity to deactivate the radio circuits repeatedly (according to the currently active DRX cycle) in order to save power. Whether the UE indeed remains in DRX (i.e. is not active) during the DRX period may be decided by the UE; for example, the UE usually performs inter-frequency measurements which cannot be conducted during the On-Duration, and thus need to be performed some other time, during the DRX opportunity of time.
The parameterization of the DRX cycle involves a trade-off between battery saving and latency. For example, in case of a web browsing service, it is usually a waste of resources for a UE to continuously receive downlink channels while the user is reading a downloaded web page. On the one hand, a long DRX period is beneficial for lengthening the UE's battery life. On the other hand, a short DRX period is better for faster response when data transfer is resumed—for example when a user requests another web page.
To meet these conflicting requirements, two DRX cycles—a short cycle and a long cycle—can be configured for each UE; the short DRX cycle is optional, i.e. only the long DRX cycle is used. The transition between the short DRX cycle, the long DRX cycle and continuous reception is controlled either by a timer or by explicit commands from the eNodeB. In some sense, the short DRX cycle can be considered as a confirmation period in case a late packet arrives, before the UE enters the long DRX cycle. If data arrives at the eNodeB while the UE is in the short DRX cycle, the data is scheduled for transmission at the next on-duration time, and the UE then resumes continuous reception. On the other hand, if no data arrives at the eNodeB during the short DRX cycle, the UE enters the long DRX cycle, assuming that the packet activity is finished for the time being.
During the Active Time the UE monitors PDCCH, reports SRS (Sounding Reference Signal) as configured and reports CQI (Channel Quality Information)/PMI (Precoding Matrix Indicator)/RI (Rank Indicator)/PTI (Precoder Type Indication) on PUCCH. When UE is not in Active time, type-0-triggered SRS and CQI/PMI/RI/PTI on PUCCH may not be reported. If CQI masking is set up for the UE, the reporting of CQI/PMI/RI/PTI on PUCCH is limited to On Duration.
Available DRX values are controlled by the network and start from non-DRX up to x seconds. Value x may be as long as the paging DRX used in RRC_IDLE. Measurement requirements and reporting criteria can differ according to the length of the DRX interval, i.e. long DRX intervals may have more relaxed requirements (for more details see further below).
FIG. 8 discloses an example of DRX. The UE checks for scheduling messages (indicated by its C-RNTI, cell radio network temporary identity, on the PDCCH) during the “on duration” period, which is the same for the long DRX cycle and the short DRX cycle. When a scheduling message is received during an “on duration”, the UE starts an “inactivity timer” and monitors the PDCCH in every subframe while the Inactivity Timer is running. During this period, the UE can be regarded as being in a continuous reception mode. Whenever a scheduling message is received while the Inactivity Timer is running, the UE restarts the Inactivity Timer, and when it expires the UE moves into a short DRX cycle and starts a “short DRX cycle timer”. The short DRX cycle may also be initiated by means of a MAC Control Element. When the short DRX cycle timer expires, the UE moves into a long DRX cycle.
In addition to this DRX behaviour, a ‘HARQ Round Trip Time (RTT) timer’ is defined with the aim of allowing the UE to sleep during the HARQ RTT. When decoding of a downlink transport block for one HARQ process fails, the UE can assume that the next retransmission of the transport block will occur after at least ‘HARQ RTT’ subframes. While the HARQ RTT timer is running, the UE does not need to monitor the PDCCH. At the expiry of the HARQ RTT timer, the UE resumes reception of the PDCCH as normal.
There is only one DRX cycle per user equipment. All aggregated component carriers follow this DRX pattern.
FIG. 9 illustrates the DRX operation of a UE having a long DRX cycle with a high number of subframes, whereas FIG. 10 illustrates the DRX operation of a UE having a long DRX cycle with a low number of subframes. As can be seen from these figures, the “short” long DRX cycle of FIG. 10 is advantageous in that the eNodeB does not have to wait too long for an opportunity to schedule the UE, compared to the long DRX cycle of FIG. 9.
Fast Dormancy
Smart phones have an increasing number of applications that only send a small amount of data, but the transmission frequency of the packets is relatively high. Such always-on applications include for example, email, instant messaging and widgets. It is important to keep UE power consumption low while having frequent transmissions. The UE could be moved to idle state for low power consumption. However, the idle state should be avoided because the next packet will then cause packet connection set-up, leading to increased latencies and signaling traffic in the network.
In order to keep UE power consumption low, the proprietary (i.e. functionality not defined by 3GPP standards) fast dormancy was introduced. When using fast dormancy, the mobile application informs the radio layers when the data transmission is over, and the UE can then send the Signaling Connection Release Indication (SCRI) to the RNC simulating a failure in the signaling connection. Consequently, the UE releases the RRC connection and moves to idle state. This approach keeps the UE power consumption low, but it causes frequent set-ups of packet connections unnecessarily increasing the signaling load. In addition, the network counters indicate a large number of signaling connection failures as this battery-saving method cannot be distinguished from a genuine signaling connection failure in the network.
3GPP Release 8 specified Fast Dormancy functionality, clarifying the UE behavior and providing the network with information of what the UE actually wants to do, but leaving the network in charge of the UE RRC state. Put differently, the UE is not allowed to release the RRC connection and move to idle on its own without network control. For further information on Fast Dormancy: WCDMA For UMTS—HSPA Evolution and LTE—Fifth Edition; Edited by Harri Holma and Antti Toskala, Chapter 15.6.
Measurements
Measurements performed by an UE are part of the Radio Resource Management and configured by the eNB. They mainly (but not exclusively) serve the purpose of handling mobility with other LTE cells or cells belonging to other Radio Access Technologies (RATs).
The Radio Resource Management procedures (specified in TS 36.133) distinguish between measurements performed in RRC_IDLE state and RRC_CONNECTED state. In the following intra-frequency measurements (i.e. the measurements on the serving cell(s) and cells located in the same frequency band), inter-frequency measurements (i.e. the measurements for cells on a frequency band different from that of the current serving cell(s)) and inter-RAT (i.e. measurements for those cells operating with other radio access technologies than UTRAN) are described in more detail with a focus on the RRC_CONNECTED state and the measurement requirements that UE has to follow when it is in discontinuous reception (DRX).
Intra-Frequency Measurements
LTE intra frequency monitoring aims at performing measurements both on the serving cell and on neighboring cells which use the same carrier frequency as the serving cell.
When DRX activity is enabled, the UE must be able to take advantage of the opportunities to save power between subsequent DRX “on periods”. Intra-frequency monitoring performance relaxations will only be defined for those cases where the periodicity of the “on period” is larger than 40 ms (Chapter 13.6.1.1 of LTE—The UMTS Long Term Evolution, Edited by: Stefania Sesia, Issam Toufik, Matthew Baker, 2009).
For measurements in the RRC_CONNECTED state when DRX is in use, the amount of intra-frequency measurements (as well as the inter-frequency and inter-RAT, see below) that are to be performed, depend on the DRX cycle length of the long DRX cycle. The below table discloses the times T_Identify and T_Measure in seconds depending on the amount of subframes of the DRX cycle.
In order to be able to perform RSRP (Reference Signal Receive Power) and RSRQ (Reference Signal Reference Quality) measurements, the UE must first synchronize to and determine the cell ID of neighbor cells; T_Identify is the time the UE is required to perform the identification of neighbouring cells it is not aware of yet; e.g. when the DRX cycle is between 40 and 80 subframes long, the UE needs to have finished the identification of neighbouring cells within 40 DRX cycles (i.e. 1.6 to 3.2 seconds). T_Measure defines how long the UE has time to perform the intra-frequency measurements on the serving and neighbouring cells; e.g. when the DRX cycle is 128 subframes, the UE needs to have finished the intra-frequency measurements within 5 DRX cycles (i.e. 0.64 seconds).
The exact timing when the cell identification and the measurements are performed, depends on the UE implementation. For example, the intra-frequency measurements which do not necessitate recalibrating the radio to another frequency (in contrast to inter-frequency measurements), the cell identification and measurements could be performed during the On-Duration where the UE is already active.
It is assumed that five measurement samples are necessary to obtain an accurate measurement results, and one subframe for every measurement sample is sufficient. See TS 36.133 Table 8.1.2.2.1.2-2 where T_Measure is indicated to be 0.2 seconds for a DRX cycle of less than 40 subframes, and 5 cycles for DRX cycles of >=40 subframes. This concludes that with a DRX cycle length of e.g. 40 subframes, the measurement will be obtained within 200 ms. The cell identification may be operated in parallel to the intra-frequency measurements.
However, cell identification might require a longer time span within each DRX cycle, e.g. 5 ms.
DRX cycle (subframes)<4040-80128>128T Identify (s)0.81.6-3.2 (40 cycles)3.2 3.2-51.2(25 cycles)(20 cycles)T Measure0.20.2-0.4 (5 cycles) 0.640.8-12.8(s) (5 cycles) (5 cycles)Inter-Frequency Measurements
LTE inter-frequency monitoring is very similar to intra-frequency monitoring. When the UE is not in DRX), the inter-frequency measurement is implemented using monitoring gaps. For a 6 ms gap pattern only 5 ms are available for inter-frequency monitoring, once the switching time has been removed; i.e. the time for the radio frequency tuning is 1 ms. UE may have 8 times (80 ms gap period) or 16 times (40 ms gap period) for measurement in 640 ms. The 5 ms stems from primary and secondary synch-channels existing every 5 ms for cell identification.
If the monitoring gaps repeat every 40 ms only 5/40=12.5% is available for inter-frequency monitoring. For this reason LTE inter-frequency maximum cell identification time and measurement periods need to be longer than for the intra-frequency case.
Within one monitoring gap the presence of the PSS (Primary Synchronization Symbol) and SSS (Secondary Synchronization Symbol) symbols is guaranteed since they repeat every 5 ms, and there is also sufficient reference signals (RS) to perform power accumulation and obtain a measurement sample for RSRP calculation. There is also sufficient signal to perform an LTE carrier RSSI (Received Signal Strength Indicator) measurement to derive RSRQ.
Similar to the intra-frequency case, if the UE is in DRX mode, some performance relaxation is required to ensure that the UE can take advantage of the DRX periods to save power. How long the UE takes only for measurement but not for identification is UE implementation dependent. The UE will use more than 5 ms for both measurement and identification. This time period for cell identification and measurement is outside the Active Time, in order to be able to retune the radio frequency part of the receiver to the other frequency.
The below table is exemplary for a gap pattern ID 0 and Nfreq=1, and discloses the time requirements for the identification of neighbouring cells and the inter-frequency measurements depending on the amount of subframes of the currently active DRX cycle.
DRX cycle (subframes)<=160256320>320T Identify (s)3.845.12 (20 cycles)6.4 10.24-51.2(20 cycles)(20 cycles)T Measure (s)0.481.28 (5 cycles) 1.6 1.6-12.8 (5 cycles) (5 cycles)Inter RAT (Radio Access Technology)
Inter-RAT measurements refer to downlink physical channels belonging to another radio access technology than UTRAN, e.g. GSM. The below table is an example for gap pattern ID 0, Nfreq=1 and UTRA_FDD.
DRX cycle (subbframes)<=406480128160>160Identify (s)2.42.563.23.23.210.24-51.2 (20(40 cycles)(40 cycles)(25 cycles)(20 cycles)cyc.)Measure (s)0.480.48 0.48 0.640.8 1.28-12.8(5 cycles)(5 cycles)Disadvantages of the Prior Art
As already explained above, the short and long DRX cycles allow a trade-off between a high battery saving and a fast response to data scheduling.
This waste of battery power is further exacerbated by the measurement and reporting requirements imposed on the UE and which depend on the long DRX cycle. As explained above, intra-frequency, inter-frequency and inter-RAT measurements are to be performed by the mobile terminal within time intervals depending on the actual DRX cycle length. Furthermore, in the active time the UE has to report SRS and CQI/PMI/RI/PTI on the PUCCH.
The intra-frequency measurements can be performed during the On-duration of the DRX operation, provided the On-duration is sufficiently long to perform said measurements, i.e. at least 5 subframes. On the other hand, inter-frequency measurements need to be performed during one of the DRX opportunities where the UE is allowed to be inactive, since the radio tuner needs to be calibrated to another frequency, which may typically take 6 subframes.
Since the measurements requirements depend on the long DRX cycle subframe length, a short DRX cycle results in that the UE has to perform the measurements more times. Taking intra-frequency measurements as an example, for a long DRX cycle length of 40 subframes, the UE has to repeatedly identify neighbouring cells within 1.6 s, and has to perform intra-frequency measurements within 0.2 s. The time requirements are relaxed for a long DRX cycle length of e.g. 2560 subframes, where the neighbouring cell identification has to be performed within 51.2 s and the intra-frequency measurements within 12.8 s (see table above).
Thus, in case the long DRX cycle is shortened to allow a reduced response time for data transmission/reception, the impact on the battery due to the On-duration and also the measurement requirements is high.
Therefore, it is important to allow a fast response time of the UE with only a minimum impact on the battery consumption of the UE.